Peptides are emerging as attractive drug delivery tools. The HIV Tat-derived peptide is a small basic peptide that has been successfully shown to deliver a large variety of cargoes, from small particles to proteins, peptides and nucleic acids. The 'transduction domain' or region conveying the cell penetrating properties appears to be confined to a small (9 amino acids) stretch of basic amino acids, with the sequence RKKRRQRRR [S. Ruben, A. Perkins, R. Purcell, K. Joung, R. Sia, R. Burghoff, W.A. Haseltine, C.A. Rosen, Structural and functional characterization of human immunodeficiency virus tat protein, J. Virol. 63 (1989) 1-8; S. Fawell, J. Seery, Y. Daikh, C. Moore, L.L. Chen, B. Pepinsky, J. Barsoum, Tat-mediated delivery of heterologous proteins into cells, Proc. Natl. Acad. Sci. U. S. A. 91 (1994) 664-668; E. Vives, P. Brodin, B. Lebleu, A truncated HIV-1 Tat protein basic domain rapidly translocates through the plasma membrane and accumulates in the cell nucleus, J. Biol. Chem. 272 (1997) 16010-16017; S. Futaki, T. Suzuki, W. Ohashi, T. Yagami, S. Tanaka, K. Ueda, Y. Sugiura, Arginine-rich peptides. An abundant source of membrane-permeable peptides having potential as carriers for intracellular protein delivery, J. Biol. Chem. 276 (2001) 5836-5840.]. The mechanism by which the Tat peptide adheres to, and crosses, the plasma membrane of cells is currently a topic of heated discussion in the literature, with varied findings being reported. This review aims to bring together some of those findings. Peptide interactions at the cell surface, and possible mechanisms of entry, will be discussed together with the effects of modifying the basic sequence and attaching a cargo.